1. Field of the Invention
The present invention relates to automated methods of measuring chemical reactions. Specifically, the invention provides methods of using an automated reflectance-reading device to determine an initiation time point and a variable end point for measuring a chemical reaction.
2. Background Information
Patients with diabetes mellitus have an impaired ability to regulate glucose levels in their blood. As a result, diabetics can have abnormally high blood sugar levels known as hyperglycemia. Over time, chronic hyperglycemia may lead to long-term complications such as cardiovascular disease and degeneration of the kidneys, retinas, blood vessels and the nervous system. Studies indicate, however, that diabetics can substantially reduce such long-term complications by rigorously managing their blood sugar levels.
A significant part of diabetes management involves monitoring blood sugar levels. Depending on the nature and severity of an individual patient's disease, the patient's blood sugar levels can fluctuate throughout the day, so that some patients measure their blood sugar levels as often as seven times a day. Consequently, patients perform much of the monitoring themselves.
Several devices are available to allow diabetics to measure their blood sugar levels at home. To use such a device, a patient typically begins by inserting a test strip into the device. Some test strips have several layers, for example a top layer to receive a blood sample and filter red blood cells from the sample, an intermediate layer containing buffers to maintain the pH of the sample, and a bottom layer containing chemical reagents to react with the sample.
After inserting the test strip into the device, the patient then applies a blood sample to the strip. When a sample is applied to the top layer of a multilayer test strip, the sample must travel by wicking action through several layers before reaching the bottom layer where the chemical reaction can occur. The device monitors the chemical reaction occurring at the bottom of the test strip and subsequently displays a calculated measurement of the patient's blood sugar level. Because such critical measurements are often self-administered at home, the operation of the device must be easy-to-use, accurate and fast. Consequently, it is important that the device use a method for measuring a chemical reaction that is rapid without sacrificing accuracy.
Two important parameters of measurement methods used in home test devices are the initiation time point of the reaction and the end point of the reaction. The initiation time point is the time from which the device begins to measure the chemical reaction. The end point is the time at which the device determines that the chemical reaction is sufficiently complete to accurately calculate the patient's blood sugar measurement.
Some home test devices rely on essentially mechanical signals for determining the initiation time point. For example, one known device begins monitoring as soon as the test strip is inserted into the device. Another known device begins monitoring only after a door on the device is closed, enclosing the test strip within the device. Thus, how quickly a patient inserts the test strip or closes the device door after applying the blood sample can affect the initiation time point, and consequently affect the consistency and accuracy of the final measurement.
Other devices begin monitoring once they detect that the test strip has become wetted with the blood sample. With multilayer test strips, however, such a device may determine an initiation time point when it detects the wetting of the top layer, despite the delay between the wetting the top layer and when the sample reaches the bottom reagent layer to begin the chemical reaction. As a result, applying the initiation method of prior art devices to multilayer test strips may not accurately determine the actual starting time of the chemical reaction. Moreover, the starting time of the reaction may also be affected by the wicking properties of the individual layers, for example, variations in the thickness and porosity. Variations in the sample itself, for example, viscosity or sample volume can also affect wicking, resulting in inconsistent initiation time points.
Such home test devices also determine an end point of the chemical reaction based on a fixed time period after the initiation time point. Applying such methods to multilayer test strips may result in further variability: if the initiation time points are determined inconsistently, the fixed end points will also be determined inconsistently, resulting in potentially inaccurate blood sugar measurements.
Therefore, presently available methods for determining initiation and end points may yield inaccurate results when applied to multilayer test strips because they rely on signals or time periods unrelated to the behavior of the chemical reaction itself. Moreover, presently available methods can require an unduly prolonged time to provide a blood sugar measurement. While saving several seconds during each measurement may not appear significant at first, the time saved is substantial over several daily measurements for the entire course of a patient's diabetes management. Furthermore, an accurate device that saves a patient's time will encourage patients to monitor their blood sugar levels more regularly, thereby promoting compliance with their prescribed regimens for diabetes management.
The present invention satisfies the need for rapid and accurate determinations of initiation and end points for measuring a chemical reaction, and provides related advantages as well.